1. Field of the Invention
The present invention relates to an optical transmission system that causes a light beam to propagate through a space and more particularly to an optical transmission system of the type needing high signal quality.
2. Description of the Background Art
Japanese Patent Laid-Open Publication No. 6-75176 discloses a microdynamic optical switch including an input port implemented by a single optical fiber, a mirror for steering a light beam, a piezoelectric actuator for moving the mirror, and an output port implemented by n optical fibers. The input port and output port are inclined by 90° relative to each other. A light beam output from the input port is incident to the output port via the mirror, which is inclined by 45° relative to an optical path assigned to the light beam. The distance between the input port and the output port is greater than 50 μm if optics includes a collimator lens or 50 μm or below if otherwise. The distance of 50 μm or below is, according to the above document, necessary for achieving a small insertion loss. Also, the document describes that the collimator lens collimates the light beam to thereby reduce the divergence of the light beam, realizing a small insertion loss even if the distance is greater than 50 μm.
However, the piezoelectric actuator taught in the above document and having a micromechanical structure cannot drive the mirror by more than a certain distance, limiting the number of input ports and output ports available. Further, the distance between the input port and the mirror and therefore the beam radius differs in dependence on the output port. This means that signal quality is dependent on the output port.
Japanese Patent Laid-Open Publication No. 2000-10029 proposes an optical switch including an input port and an output port implemented by optical fibers, a plurality of lenses, a plurality of movable mirrors, and a fixed mirror. This document describes that the movable mirrors so operate as to minimize an optical loss. Such an optical switch, however, needs the movable mirrors and requires them to move over a broad range while increasing a distance between the input port and the output port. The increase in the distance between the input port and the output port causes the distance between the input port and the mirrors to vary independence on the output port, also causing signal quality to vary in dependence on the output port.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 11-2769, 11-90665, 11-95154 and 2001-174724 and Japanese Patent No. 3,040,812.